Security cards such as identity cards and credit cards conventionally comprise a data-carrying sheet of, for example, paper or card, protected by a layer of a transparent plastics material. The latter is usually applied to the data-carrying sheet by a laminating process. The plastics material is usually a thermoplastic material and adhesion between the data-carrying sheet and the plastics material is obtained either by the use of a hot-melt adhesive or by relying on melting of the surface of the plastics material during the laminating process. These conventional methods of effecting adhesion between the data-carrying sheet and the plastics material have not proved satisfactory because, after heating, these layers of the card can be delaminated without destroying the data-carrying sheet. Similarly, in conventional methods of producing a security device incorporating a hologram, a hologram comprising a film of polymeric material having thereon a layer of gelatin bearing a holographic image is bonded to another component of the device using a hot melt adhesive. Again these conventional methods have not proved satisfactory since holograms bonded to security cards by hot melt adhesives can be removed after moderate heating without destroying the hologram or the security card.
In European Patent Publication EP-A-0273012, it has been proposed to bond a gelatinous surface such as that of a hologram to another surface using an adhesive comprising a carboxylic acid having at least one polymerisable acrylic group. It is indicated that the adhesive can be liquid or solid (including a solid preformed film). While liquid adhesives in accordance with EP-A-0273012 have been used successfully, the formulation and use of film adhesives has proved more problematic, so that with film adhesives it is frequently difficult to achieve the desired bond strengths.
It has now been found that good bond strengths between component layers of data-carrying laminates at temperatures up to 135.degree. C. can be achieved by the use of curable solid self-supporting film adhesives comprising a solid polymer of a vinyl carboxylic acid and a curable material having at least one acrylate or methacrylate group. It has also been found that it is not necessary to include in the film adhesive a carboxylic acid having a polymerisable acrylic group which is the essential component of the adhesive described in EP-A-0273012, thereby avoiding the problems which can arise in some instances because of the corrosive nature of such carboxylic acids. When component layers of a data-carrying laminate are bonded using a curable solid self-supporting film adhesive comprising a solid polymer of a vinyl carboxylic acid and a curable material having at least one acrylate or methacrylate group, the film adhesives can be cured by photopolymerisation of thermal polymerisation to give a bond strength such that the component layers of the laminate cannot be separated without destroying the layers themselves.